1. Field of the Invention
The present invention pertains generally to methods used in lithographic etching, more specifically to high aspect ratio anisotropic etching with smooth etched sidewalls.
2. Description of the Relevant Art
Etching of Si surfaces using halogens plasmas is a central process in the microstructuring of semiconductors. There has been an extensive effort over the past decade to move from the microstructure regime to the nanostructure regime to develop nanoelectronic devices, Nano Electro Mechanical Systems (NEMS), and nanoresolution templates for Nano Imprint Lithography (NIL).
Dry etching processes must fulfill a number of requirements: adequate etch profile and selectivity control with respect to the mask and materials beneath the mask to be etched, no significant substrate damage, and sufficiently high etching rates. These requirements become even more stringent when etching nanoresolution devices due to the extremely small dimensions involved.
Generally, high-resolution anisotropic silicon (Si) etching is performed using high density, low-pressure plasma discharges employing halogen gases comprising bromine (Br) or chlorine (Cl). Etching Si with Cl and Br requires both the selective transport of low energy reactive species and bombardment by highly energetic particles. A large body of literature explains many of these underlying etching mechanisms.
Unfortunately, bombardment by energetic particles induces complications due to radiation damage, undercutting, and contamination. Some of these issues, such as radiation damage from ultraviolet and X-ray photons [J. R. Woodworth, M. G. Blain, R. L. Jarecki, T. W. Hamilton, 1449 B. P. Aragon, J. Vac. Sci. Technol. A17 (1999) 3209] and charge-build-up of positive ions and electrons [T. Nozawa, T. Kinoshita, Jpn. J. Appl. Phys. 34 (1995) 2107] can result in detrimental device performance. These are very serious problems to overcome for manufacturing of future nano-scale devices (devices with characteristic dimensions typically measured in nm, or 10−9 m.
Fluorine's spontaneous reaction with silicon is generally advantageous, because no high-energy ion bombardment is necessary for etching, thereby minimizing lattice damage. Unfortunately, this spontaneous Si+F reaction makes anisotropy control a very difficult issue.
A gas chopping process is capable of controlling the anisotropy by alternating steps of sidewall passivation with etching steps. In combination with an inductively couple plasma (ICP) reactor, gas chopping offers the unique opportunity to control the energy and density of the ion and neutral fluxes almost independently, with minimum overlap between the etching and passivation steps as the precursors are switched. Consequently, gas chopping has become a very important process for MEMS fabrication.
Extending gas-chopping technology to the sub-50 nm regime is challenging. Gas chopping typically results in a roughness, scalloping, or rippling of the sidewalls. In the gas chopping technique, periodically changing the supply of deposition and etch precursors produces overall anisotropically etched trenches, however, the sidewall surfaces of etched features are typically rippled. Each ripple represents a single deposition and etching cycle.
To improve the sidewall ripple problem, it has been found essential to generate certain radicals in fluorocarbon plasmas. The dependence of anisotropy on etching conditions in a gas chopping deep reactive etching technique GChDRIE has been described in previous publications [B. E. Volland, Tzv. Ivanov, and I. W. Rangelow, J. Vac. Sci. Technol. B 20 (2002) 3111].
To achieve smoother sidewall structures a modified Gas Chopping Deep Reactive Ion Etching (GChDRIE) process has been developed [B. Volland, F. Shi, P. Hudek, H. Heerlein, and I. W. Rangelow, J. Vac. Sci. Technol. B 17 (1999) 2768]. In this process the isotropic etching step is replaced by an anisotropic etching step. When this technique is combined with ICP and fluorine chemistry, it provides high etching rates and satisfying sidewall slope control. By increasing the gas-chopping frequency, the amplitude of the ripples are reduced to the 10 nm range, but this is still too large for sub-50 nm structures. While increasing the chopping frequency still further is theoretically possible, in practice this cannot be achieved due to the relatively long residence times of gas precursors and products, coupled with limited pumping speeds.
Therefore, there is a need in the area of plasma etching for optimizing the performance of gas chopping etching processes to be able to transfer anisotropically etched nano-features in the sub-50 nm regime.